1. Field of the Invention
The present invention relates to LED driving circuits, and more particularly to open loop LED driving circuits capable of delivering constant average current to a set of LEDs, irrespective of line voltage variations.
2. Description of the Related Art
In supplying power for a LED (Light Emitting Diode) module, open loop switching power conversion architecture is widely adopted due to cost issue. The open loop switching power conversion features a concise circuit topology by eliminating the transformer and the voltage feedback network.
FIG. 1 shows the architecture of a prior art open loop LED driving circuit. As shown in FIG. 1, the architecture includes: a PWM controller 100, a capacitor 101, a diode 102, a LED module 103, an inductor 104, an NMOS transistor 105, and resistors 106˜107.
In the architecture of the prior art open loop LED driving circuit, the PWM controller 100 is used for generating a PWM signal with a duty cycle at the GATE pin in response to a current sensing voltage at the CS pin.
The capacitor 101 is used to filter out the high frequency components from a voltage source VIN to generate a DC voltage, wherein the voltage source VIN is rectified from an AC line voltage.
The diode 102 is used for releasing the magnetic flux in the inductor 104 to drive the LED module 103.
The LED module 103 is the load of the open loop LED driving circuit.
The inductor 104 is used for carrying the magnetic flux to provide a current ILED to drive the LED module 103.
The NMOS transistor 105 is used to control the magnetic flux transformation through the inductor 104 in response to the PWM signal at the GATE pin. When the NMOS transistor 105 is during a turn-on period, the LED module 103, the inductor 104, the NMOS transistor 105, and the resistor 106 will constitute a conduction path to store the magnetic flux in the inductor 104; when the NMOS transistor 105 is during a turn-off period, a conduction path composed of the LED module 103, the inductor 104, and the diode 102 will be formed to release the magnetic flux from the inductor 104.
Through a periodic on-and-off switching of the NMOS transistor 105, which is driven by the PWM signal generated from the PWM controller 100, the input power from the voltage source VIN is transformed through the inductor 104 to the LED module 103 in the form of a regulated current.
The resistor 106, connected between the CS pin of the PWM controller 100 and a reference ground, is used for converting the current, which corresponds to the magnetic flux being stored in the inductor 104, to the current sensing voltage at the CS pin when the NMOS transistor 105 is during a turn-on period.
The resistor 107, connected between the RT pin of the PWM controller 100 and the reference ground, is used to configure the PWM controller 100 to operate in a constant frequency mode.
Due to the on-and-off switching of the NMOS transistor 105, the current ILED is increasing during the turn-on period and decreasing during the turn-off period. Please refer to FIG. 2, which shows the waveform diagram of the LED driving current ILED of the prior art open loop LED driving circuit in FIG. 1, corresponding to a voltage source VIN. As can be seen in FIG. 2, when the current ILED ramping up in a turn-on period reaches Vth/Rcs, which is set by the PWM controller 100, the NMOS transistor 105 is turned off and the current ILED starts to ramp down into a turn-off period Toff. The current ILED has an up-going slope mch in the turn-on period and a down-going slope mdis in the turn-off period, with mch=(VIN−VO)/L and mdis=(VO+VD1)/L, wherein VO is the voltage dropt of the LED module 103, and VD1 is the forward voltage dropt of the diode 102. The average current of the current ILED is thus calculated as:ILED,avg=Vth/Rcs−Toff×mdis/2.
Ideally the average current ILED,avg is independent of the voltage of the voltage source VIN. However, since there exists a delay time in switching off the NMOS transistor 105, the current ILED will exceed the Vth/Rcs with an amount equal to the product of the delay time and the up-going slope mch; and since the up-going slope mch depends on the voltage source VIN, the peak current and so the average current of the current ILED will no longer remain constant when the voltage source VIN is changed to a different one. Besides, if the capacitance of the capacitor 101 is reduced or the capacitor 101 is just simply removed due to cost reduction issue, the voltage sources corresponding to different AC line voltages will exhibit unfiltered waveforms and cause more severe variations to the average current ILED,avg. The reason is that when the voltage source VIN shows an unfiltered full-wave rectified waveform, there will be a dead time in the conduction of the diode 102, and the dead time, inversely proportional to the voltage of the line voltage, will reduce the average current ILED,avg. Please refer to FIG. 3, which shows the waveform diagram of a LED driving current ILEDL corresponding to a higher voltage source VINH compared with another LED driving current ILEDL corresponding to a lower voltage source VINL. As can be seen in FIG. 3, the dead time of the higher voltage source VINH during when the voltage of VINH is under VO is shorter than that of the lower voltage source VINL, so the average current of ILEDH is larger than that of ILEDL. That is, line voltage variations can cause luminance variations of the LED module.
Therefore, there is a need to provide a robust solution for the open loop LED driving circuit to regulate the driving current against line voltage variations.
Seeing this bottleneck, the present invention proposes a novel open loop LED driving circuit, capable of adaptively adjusting the turn-off period in response to line voltage variations to regulate the driving current.